1. Field of the Invention
The present invention relates to a magneto-optical recording medium, and particularly to a magneto-optical recording medium having such a reflective film as to give excellent CN ratio (carrier-to-noise ratio) and recording sensitivity.
2. Prior Art
Magneto-optical recording media (hereinafter sometimes referred to in brief as "recording media") as high-density recording media having a magnetic film adapted to rewriting are under active investigation and development.
Among magneto-optical recording media constituting the magnetic films of such recording media, amorphous alloys of a rare earth metal with a transition metal (hereinafter sometimes referred to as "RE-TM alloys") have been most progressed in studies and most put into practical use since they are formed into a perpendicular anisotropy film with direction of magnetization oriented pependicularly to the surface of the film, have a large coercive force of several KOe, and are capable of being relatively easily formed into a film by a deposition technique such as sputtering, or vacuum evaporation and deposition.
Since recording media produced using an RE-TM alloy have a perpendicular anisotropy film as the magnetic film thereof, they have excellent features of being capable of recording information up to an extremely high density of 10.sup.8 (bits/cm.sup.2) and being, in principle, capable of undergoing infinite repetition of erasing and rewriting information.
However, a magnetic film made of an RF-TM alloy is defective in that it has a poor corrosion resistance [see "Hikarijiki Disk" complied under the general editorship of Nobutake Imamura and published by K. K. Triceps, p. 427 (Literature I)] and exhibits only a little magneto-optical effect (Kerr effect).
Thus, there are known structures of recording media which comprise a magnetic film as mentioned above and a reflective film provided on the side of the magnetic film opposite the reading side thereof and/or protective films provided so as to sandwich the magnetic film to increase the apparent Kerr rotation by utilizing the refraction or reflection of light (see the above-mentioned Literature I, p.119).
The above-mentioned conventional magneto-optical recording media will now be described while referring to the accompanying drawings.
FIG. 1(a) is a schematic cross-sectional view of an example of the conventional recording media, which is illustrative of the structure thereof.
As will be understood from FIG. 1(a), a protective film 13a, a magnetic film 15, a protective film 13b and a reflective film 17 are formed in this order on the upper surface of a substrate 11 to produce a recording medium 19.
The substrate 11 is made of a material which is transparent at the wavelength of a light to be used in writing in or reading from the recording media 19. Examples of such a material include polycarbonate resins, glass, and epoxy resins.
The protective films 13a and 13b are formed by deposition of SiO, SiO.sub.2, AlN, Si.sub.3 N.sub.4, AlSiN, AlSiON, or other protective film material.
The magnetic film 15 is made of an RE-TM alloy as mentioned above, known examples of which include Tb-Fe alloys, Tb-Co alloys, Tb-Fe-Co alloys, and the like.
Aluminum (Al) is most widely used as the material of the reflective film 17, other examples of which include gold (Au), copper (Cu), and titanium (Ti).
As another known recording medium having a reflective film 17 as mentioned above, a recording medium 21 as shown FIG. 1(b) is produced by forming on the upper surface of a substrate 11, a protective film 13a, a magnetic film 15, a reflective film 17 and a protective film 13b in this order.
Information writing on such a recording medium is carried out according to the so-called thermomagnetic writing system in which the recording medium to which an external magnetic field is applied is irradiated with a laser beam having a fined spot diameter of about 1 .mu.m in such a direction that the beam advances from the substrate 11 to the magnetic film 15. More specifically, the magnetic film 15 locally heated with the above-mentioned laser beam is lowered in coercive force in the heated area thereof, wherein information is immediately written by the external magnetic field carrying information for recording. Information writing may also be carried out by means of a pit length and/or an interval between pits created by a laser beam as mentioned above.
As can be understood from the foregoing description, the recording sensitivity of a magneto-optical recording medium is greatly affected by the heat retaining properties in the magnetic film thereof and the degree of multipath reflection.
Accordingly, when the reflective film is viewed from the foregoing point of view, the reflective film is required to be made of a material having not only such a low thermal conductivity as to suppress heat dissipation at the time of information writing but also such a high reflection factor as to give efficient multipath reflection at the time of information reading.
As described above, when silver (Ag), among known materials of reflective films, is used to form a reflective film, a CN ratio of about 48 dB can be attained because Ag has a high reflection factor. On the other hand, however, the high thermal conductivity of Ag makes it necessary to use a high energy as a recording power corresponding to the output of a laser beam for use in information writing in order to make up for large heat dissipation from a magnetic film.
In view of the foregoing problem of the prior art, an object of the present invention is to provide a magneto-optical recording medium which not only has a practical readout sensitivity, but also permits information writing therein to be carried out with a smaller recording power than a magneto-optical recording medium having a reflective film made of silver alone.